1. Field of the Invention
The present invention relates to an improved technique for embedding a digital watermark in digital contents information during an encoding process, which watermark is durable against degradation and more undecipherable to a third party, and a technique for detecting unauthorized use during the decoding process using the digital watermark embedded in the compressed code stream.
2. Description of Related Art
Digital information (or digital contents), such as digital images, can be copied easily by computers, without degrading the information. In recent years and continuing, along with the progress in digital image technologies, digital photographs taken by a digital camera are likely to be used as evidence. There is some digital information protected by copyright, and unauthorized copy or reuse of the copyrighted digital contents is prohibited.
In general, digital contents are easy to copy, and easy to tamper with by rewriting or overwriting the contents through simple modification processes. For this reason, digital contents are often copied dishonestly, and the copied contents are willfully misappropriated, destroyed, or tampered with.
To overcome this problem, a technique called digital watermarking or data hiding has been proposed, which is used to prevent unauthorized copying by embedding information into digital contents (such as digital images). The embedded information is invisible when the digital contents are reproduced in an ordinary manner. By checking the digital watermark embedded in the digital contents, it can be determined whether the digital contents have been used dishonestly.
A digital watermark embedded in digital contents has several uses, such as    A. Recording copyright information,    B. Tracking information about illegal copying,    C. Providing a past record of IP addresses,    D. Preventing illegal copying (invisible and high-durability type),    E. Application to anti-tampering (invisible and low-durability type),    F. Authentication,    G. Secure (encrypted) communications,    H. Embedding notes or labels of digital contents for indicating the owner (either visible or invisible type), and    I. Enabling removal of the watermark (visible or invisible type for distributing contents).
Among these, systems for preventing illegal copying (above-described item D) have been proposed for the purpose of protecting the copyright of a creator of digital contents.
For example, it is proposed, when selling or distributing digital contents regarding copyright related information, such as information about the copyright owner and the purchaser himself/herself in the digital contents, to let the contents provider of the digital contents inform the purchaser about the digital watermark representing the copyright related information in order to allow the purchaser to know about the copyright. This system aims to psychologically inhibit a person with common sense from making improper use of the digital contents. Although a malicious user may ignore the copyright and make an unauthorized use, it is possible for the copyright owner to identify the malicious user and assert the copyright by acquiring the illegally used digital contents and extracting the digital watermark from the acquired digital contents. See “Technical Report about Digital Watermarking”, March, 1999, Nippon Denshi Kogyo Shinko Kyokai. The original source is “Prevention of Illegal Copying of Electronic Information”, Nikkei Business, Feb. 23, 1998, at 68-70 (1998).
Techniques for embedding digital watermarks in digital contents (hereinafter, referred to as “digital watermarking”) are grouped into (1) embedding a digital watermark directly in sampling values of the contents data, and (2) embedding a watermark in frequency components.
With the former technique (for embedding a watermark directly into sampling values), the process workload for processing and compressing the digital contents is not so heavy. However, when performing digital processing and compression of the digital contents in which a watermark is embedded, the digital watermark is likely to be lost.
On the other hand, the latter technique (for embedding a watermark in frequency components) requires heavy processing for embedding and extracting the digital watermark into and from the digital contents. However, it is durable against digital processing and compression.
Meanwhile, for monochromatic digital photocopy, facsimile, and monochrome print of a newspaper, lossless image compression using black-and-white bi-level images and capable of restoring the original image without loss is recommended and widely used. As lossless image compression schemes, MH/MR/MMR under G3/G4 standards of the International Telecommunications Union Telecommunication Standardization Sector (ITU-T) are known.
However, these schemes are inferior in coding efficiency for intermediate processed images or error-diffusion processed images, and accordingly, image compression may not be correctly performed.
To overcome this problem, a method called arithmetic coding, which a type of entropy coding, is attracting attention. For instance, JBIG, which is a group for investigating a new international standard for a coding scheme of bi-level images, employs arithmetic coding called QM-coder, which is recommended as an ITU-T standard.
In contrast, those products that handle multi-level images including digital color images or gray-scale images (e.g., digital still cameras and digital video cameras) often employ lossy compression. Lossy compression is a technique that gives priority to the compression effect when storing data, while making a sacrifice of slight degradation of image quality due to loss of detailed information of the original image. Examples of the lossy compression technique include JPEG and MPEG, which are based on signal-to-frequency conversion using discrete cosine transform (DCT).
As the post-JPEG standard, JEPG 2000 is attracting a great deal of attraction as the coding standard unifying lossless and lossy coding schemes, as well as unifying bi-level data and multi-level data. JPEG 2000 employs discrete wavelet transform (DWT) for converting a signal into elementary frequency components to reduce image degradation during high-rate compression. In addition, MQ-coder is used to achieve high-rate compression. MQ-coder is an arithmetic coding scheme similar to the above-described QM coder, and is approved by JBIG.
The above-described former (first-listed) digital watermarking technique (i.e., watermarking directly in the sampling values of the digital contents) is unsuitable for digital contents of multi-level images to be subjected to compression, because the image quality is degraded due to encoding and compression of images performed when storing the digital contents. Due to the degradation of the images, the digital watermark embedded in the digital contents is also degraded. As compression encoding and decompression decoding are repeated, the distinguishability of the digital watermark embedded in digital contents weakens. In the worst case, it becomes difficult to detect unauthorized use of the digital contents in spite of the fact that the digital watermark is embedded in the contents.
To this end, it is preferable to employ the above-described latter technique for embedding a watermark in the frequency component (or the code stream prepared for compression) when the digital watermark is used for the purpose of preventing unauthorized use of digital contents, so that the digital watermark can be correctly detected even after encoding and decoding processes are repeated.